This invention relates generally to the rendering of latent electrostatic images visible using multiple colors of dry toner or developer and more particularly to a developer apparatus and its method of operation wherein the transient development stress problems related to the xerographic start-up and shut-down of a single-pass tri-level xerographic engine as well as other problems are overcome.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not struck by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
The concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.cad or V.sub.ddp), the other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.dad or V.sub.c (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD). The background areas are exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 v closer to V.sub.cad than V.sub.white (about 600 v), and the DAD developer system is biased about 100 v closed to V.sub.dad than V.sub.white (about 400 v).
In the case of a tri-level xerographic engine start-up, if, for example, the CAD developer bias is switched on prior to the arrival of the sensitized and properly exposed photoreceptor, the large uncharged area of the photoreceptor would generate a large cleaning field of the magnitude of the developer bias voltage (600 v). In the presence of such a large cleaning field, developer beads would tend to be preferentially attracted to the photoreceptor, leading to a condition known as bead carry-out. The high cleaning field would be removed only with the arrival of a properly charged photoreceptor at the V.sub.white potential.
Conversely, if the photoreceptor charged to V.sub.white arrives at the CAD developer prior to the application of the developer bias voltage, the photoreceptor potential acts as a large solid-area image having a significant development potential (V.sub.white, 500 v) to the toner particles, carrying out much toner from the developer housing, contributing to machine dirt and subsequent reliability problems, with the possibility of overloading the cleaner, and, because the development is carrying out toner faster than the developer system can replenish the carrier beads, also possibly causing a bead carry-out problem.
The case of DAD developer under similar conditions causes high toner carryout if the bias (400 v) is applied prior to the arrival of the photoreceptor at the V.sub.white potential, and possible bead carryout if the bias is applied subsequent to the arrival of V.sub.white.
The foregoing and other problems are solved by my invention which includes the provision of developer structure comprising a plurality of rolls adapted to be rotated in one direction for developing latent images and in the reverse direction in order to remove the developer from the development zone.
Removal of developer from the development zone for various reasons is taught in the prior art. For example:
U.S. Pat. No. 3,940,272 granted to James R. Davidson on Feb. 24, 1976 discloses a method of developing electrostatic latent images in plural colors which comprises a separate developer structure for each color. As noted in column 6, lines 31-36, once the complete image recorded on the photoconductive surface has passed the first development zone development action msut be terminated in order to preclude intermingling of the different color developers used for the three different images developed. To effect such termination, rotation of a paddle wheel, transport roll and a developer roll comprising the development structure is stopped. This permits that development housing to pivot away from the photoconductive surface so that the developer therein ceases contact with the photoconductive surface.
U.S. Pat. No. 3,709,713 granted to L. H. Turner on Jan. 9, 1973 discloses a magnetic developer system wherein the magnet of a magnetic brush roll is pivotally displaced away from a photoreceptor such that the developer carried thereby does not develop the image on the photoreceptor. The purpose of this arrangement is to eliminate unwanted development of charged but non-imaged areas such as interdocument areas in an automatic xerographic machine having solid area capabilities.
U.S. patent application, Ser. No. 844,681, cited in U.S. Pat. No. 4,352,552 and now abandoned, discloses, in a three-pass color copier, the reverse rotation of a developer roll as well as the other members of the development device for effecting the removal of the developer from the developer roll and, therefore, away from the latent image so that an image of one color is not contacted by developer of a second developer.
In U.S. Pat. No. 4,053,218, it is stated that U.S. Pat. No. 3,570,453 and 3,575,139 teach the articulation of a blade into contact with the developer roll to prevent developer mix from being moved into the development zone.
U.S. Pat. No. 3,662,395 issued to Doi et al on May 9, 1972 discloses the reverse rotation of a developer roll and as set forth in column 9, lines 65-72, a cooperating doctor plate scrapes the powder from the magnet drum.
Another invention assigned to the same assignee as the instant invention and disclosed in U.S. patent application, Ser. No. 78,750 relates to the use of programmable power supplies and a controller to minimize the electrical stress on the developer in contact with the photoreceptor during start-up and shut-down.
U.S. Pat. No. 3,392,432 assigned to Azoplate Corp. discloses a magnet roller wherein a magnetizing system and its surrounding shell are adapted to be rotated together while occupying different relative angular positions relative to each other. In one relative position, developer mix is attracted to the roller and in the other relative position it is not so attracted.
Xerox Disclosure Journal, Vol. 12 Number 2 dated March/April 1987 discloses a rapid cut-off developer system for highlight color wherein a shield can be selectively interposed between the developer and the charge retentive surface in order to collapse the magnetic field of the developer structure for removing developer from contact with the charge retentive surface.
It is of interest to note that, due to the finite width of the developer zone of a single roll, it is not possible to avoid the aforementioned conditions during start-up or shut-down with hardware that can provide only step transitions (switch on/switch off) in developer bias potentials or photoreceptor potentials.